In image display devices such as liquid crystal display devices, retardation films are usually widely used for improving viewing angle characteristics of display screens.
Although λ/2 plates and λ/4 plates are known as the aforesaid retardation films, many of them have a characteristic that absorption is greater on a short wavelength side and the retardation increases as the wavelength becomes shorter. Such a characteristic is generally called a positive wavelength dispersion characteristic (henceforth referred to as “positive dispersion”). However, retardation films which exhibit positive dispersion have some problems such as those shown below.
The retardation of a retardation film usually is adjusted to ½ of a wavelength in a case of a λ/2 plate and to ¼ of a wavelength in a case of a λ/4 plate. Ideally, a characteristic curve of the retardation film with wavelength as an abscissa and retardation as an ordinate is demanded to show a straight line with a shift rising steadily from left to right. This is because if such a characteristic curve is shown, the retardation increases with increase in wavelength and, therefore, retardations near values of ¼ or ½ of a wavelength are obtained for any wavelength. However, a retardation film with a positive dispersion actually shows a curve with a shift rising steadily from right to left, which behaves differently from an ideal straight line, because such a film exhibits a larger retardation as the wavelength becomes shorter as described above. In other words, although a desired retardation is satisfied for some wavelengths, it is impossible to obtain a desired retardation in a wide wavelength band. For this reason, it is difficult for retardation films with a positive dispersion to convert light into linearly polarized light over a wide wavelength band.
For such a reason, retardation films which exhibit wavelength dispersion characteristics different from positive dispersion have recently been attracting attentions. They are retardation films which exhibit characteristics such that the retardation increases as the wavelength becomes longer, namely, wavelength dispersion characteristics with reverse dispersion (henceforth, referred to as “reverse dispersion”). In such retardation films, the retardation becomes larger as the wavelength becomes longer. Therefore, the aforesaid characteristic curve of the retardation shows a curve with a shift rising steadily from left to right and approximates an ideal behavior. In other words, in the case of a λ/4 plate for example, a retardation near a value of ¼ of a wavelength is obtained in a wide wavelength band. Therefore, it becomes possible to convert light into polarized light as a λ/4 plate in a wide wavelength band. In a case of a retardation film which exhibits a larger reverse dispersion, it can be used in a wide wavelength band as a λ/2 plate whose ideal retardation is a value of ½ of a wavelength. Regarding the “amplitude of reverse dispersion”, in the aforesaid characteristic curve for example, the relatively greater the slope, the larger the reverse dispersion; whereas the relatively smaller the slope, the smaller the reverse dispersion. Therefore, in the case of a λ/2 plate, what is required is that the slope of the aforesaid characteristic curve is larger (a larger retardation is shown for every wavelength), in other words, there is a large reverse dispersion, in comparison to a λ/4 plate, because ½ of a wavelength is the ideal retardation of the λ/2 plate.
The reverse dispersion characteristics mentioned above usually depend upon a kind of the polymer used as a raw material of the retardation film. However, there are very few reports about polymer which can realize reverse dispersion.
Specifically, for example, a method has been reported in which a retardation film with a reverse dispersion is formed by forming a film from a polymer resulting from polymerization of two kinds of monomers and drawing the film, thereby causing the film to express retardation (JP-A 2002-221622). The two kinds of monomers in this method are of a combination in which one shows a positive birefringent property (monomer 1) and another shows a negative birefringent property (monomer 2), wherein the wavelength dispersion characteristics of both monomers satisfy monomer 1<monomer 2. On the other hand, a method has been reported in which a retardation film with a reverse dispersion is formed by blending two kinds of polymers differing in retardation polarity and wavelength dispersion characteristics (JP-A 2002-14234). Further, a method for producing a retardation film with a reverse dispersion from a mixture of a liquid crystal molecule and a polymer has also been reported (JP-A 2002-48919).
However, a polycarbonate having a fluorene skeleton disclosed in the above-cited JP-A 2002-221622 has a very high glass transition point due to its structure. Therefore, there is a problem that the drawing temperature must be set at a very high temperature in a drawing treatment for expressing retardation. In addition, when such an undrawn film of polycarbonate is subjected to shrinking treatment in order to increase a refractive index in a thickness direction, there also arises a problem described below. The shrinking treatment is a method in which an undrawn film is stuck to a film capable of shrinking on heating and the resulting laminate is heated and drawn (JP-A 5-157911). In this case, it is difficult to produce a retardation film with a high refractive index in the thickness direction in an industrial scale because the drawing temperature of the undrawn film of polycarbonate is too high in comparison to the shrinking temperature of the shrinking film.
The method disclosed in JP-A 2002-14234 has a problem that when dissolving two kinds of polymers together it is difficult to maintain transparency of a resulting blend polymer and alternatives of the combination of the polymers are limited. Moreover, also in the method disclosed in JP-A 2002-48919, it is difficult to choose a combination of a polymer and a liquid crystal molecule which are compatible with each other. For example, in some combinations, liquid crystal molecules dispersed in a polymer change into liquid during the heating and drawing treatment of a film. This may cause the resulting retardation film to have a high haze, which may result in reduction in transparency.